Power generators using propellers have been known that generate power by the propellers rotated by fluids such as wind and ocean currents. For example, in wind power generators, power generation units convert mechanical energy of the propellers rotated by wind into electrical energy.
Some power generators using propellers include position detectors such as encoders that detect rotational positions of the propellers. An example of related art power generators includes such a position detector attached to a propeller shaft and the position detector detects the rotational position of the propeller by detecting the rotational position of the propeller shaft (e.g., refer to Japanese Patent Application Laid-open No. 2011-208635).